In many Western countries, flood policy is transitioning from a focus on technical flood defence measures towards more holistic and integrated flood risk management approaches. In this article, we explore the boundary spanning role of landscape architects in integrated flood risk management projects. The central research question is: what are the boundary spanning activities and roles that landscape architects perform and which factors are conditional to these activities? We have studied the boundary spanning behaviour of landscape architects in the Dutch 'Room for the River' programme. This programme had a dual objective of improving simultaneously the water safety and the spatial quality of the Dutch riverine areas. We conducted a comparative, in-depth case study of three 'Room for the River' projects, and investigated conditions that stimulated or frustrated the work of landscape architects in establishing safe solutions with spatial quality. We found that the landscape architects involved in these projects played various boundary spanning roles. We conclude that, depending on the conditional factors, their roles ranged from more traditional content-oriented domain expert/scout to the more innovative organisational expert/task coordinator. For successful boundary spanning, although cognitive capacities (e.g., knowledge about landscape) are important, landscape architects also need to have the appropriate social capacities (e.g., social-emotional competences, networking skills). That is, the work of the landscape architects essentially includes drawing lines that sketch the contours of future landscapes; but to do so, they must also cross the lines between the various actors, organizations, and disciplines involved.